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infection can occur. This knowledge suggests that aerosols may play a 
major role in the causation of laboratory acquired infections. 
Physical containment is primarily used to prevent the release of aero- 
sols. Containment is dependent on technique, specially designed physical 
containment apparatus such as safety blenders, and biological safety cabi- 
nets. Technique can minimize, but not prevent, the release of aerosols. 
Containment apparatus and safety cabinets can be designed to reduce or 
eliminate exposures to aerosols released during the conduct of microbiolog- 
ical procedures. 
The principal device for containing hazardous microorganisms is the 
biological safety cabinet. I will provide a description of biological 
safety cabinets and then discuss the various combinations of technique, 
cabinets, and facility features which comprise the four levels of physical 
containment (PI, P2, P3 and P4) proposed for recombinant DNA research. 
Biological Safety Cabinets suitable for confining operations involving 
recombinant DNA molecules are classified as Class I, Class II, and Class III 
Cab inets. 
The Class I cabinet is a ventilated cabinet for personnel protection 
only, with an unrecirculated inward flow of air away from the operator. The 
exhaust air from this cabinet may be filtered through a high-efficiency or 
high-efficiency particulate air (HEPA) filter before being discharged to the 
outside atmosphere. This cabinet is suitable for research work with the 
Center for Disease Control (CDC) classes of etiologic agents 1, 2, and 3 
where no product protection is required. This cabinet may be used in three 
operational modes; (1) with an 8-inch high, full-width open front, (2) with 
an installed front closure panel (having four 8-inch diameter openings) 
without gloves, and (3) with an installed front closure panel equipped with 
arm-length rubber gloves. 
The Class II cabinet is a ventilated cabinet for personnel and product 
protection having an open front with inward air flow for personnel protec- 
tion, and HEPA-f iltered recirculated mass air flow for product protection. 
The cabinet exhaust air is filtered through a HEPA filter. Two models of 
this cabinet are available, Type 1 and Type 2. The Type 1 recirculates 
approximately 70 percent of the air. The exhaust air from this cabinet may 
discharge into the laboratory or be diverted out of the laboratory. This 
cabinet is suitable for CDC classes of etiologic agents 1, 2 and 3. Vapors 
or gases which are hazardous from a toxic, radioactive, or flammability 
standpoint should not be used in this cabinet because of the high quantity 
of recirculated air. The Type 2 cabinet recirculates approximately 30 
percent of the air. The exhaust air from this cabinet is normally ducted 
out of the laboratory through a HEPA filter and, occasionally, an activated 
charcoal filter depending on the operation. The cabinet may be used with 
gases or vapors that are hazardous from a toxic, radioactive, or flammability 
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